The present invention relates to an optical pickup device which is used to record information on or reproduce information from an information recording medium such as DVD (Digital Video Disc or Digital Versatile Disc) and CD (Compact Disc).
When an optical pickup is used to record information on or reproduce information from an information recording medium such as a DVD or a CD, a light beam is emitted from a light source and is then converged by an object lens so as to be formed in to a recording beam for recording information on a disc or a reading beam for reading information from a disc. In fact, such a recording beam or a reading beam is required to be focused on a recording surface of a disc when being incident thereon.
In order to detect whether or not a desired focused state has been reached and to detect whether or not there is a deviation from the desired focused state, a focus servo control is performed in which a focus error signal is generated (if there is such a deviation) and is used to control the focusing operation so that the focal point of the objective lens may be constantly and optically set on the recording surface of a disc (hereinafter referred to as "focused state").
As typical methods for generating a focus error signal, there have been known an astigmatism method and a spot size method.
In the astigmatism method, a light reflected from an information recording medium, upon being separated from an original light beam (emitted from a light source and irradiating the recording medium) in a beam splitter, is passed through a cylindrical lens so as to produce an astigmatic difference. The pattern of a reflected light beam reflected from the recording surface is detected in accordance with a distance between the objective lens and the recording surface, thereby producing a focus error signal.
In detail, the reflected light bean is detected by a photodetector having a 4-divided light-receiving area, when the objective lens is in its correct focusing position, a true circular pattern of a reflected light will come to the central position of the light-receiving area.
If a distance between the objective lens and a disc is shorter than a predetermined focal distance, a reflected light having a horizontally arranged elliptical shape will come to the light-receiving area, and thus a difference between the output signals fed from different sections of the light-receiving area may be calculated, thereby producing a focus error signal (indicating a so-called "front pin state" in which the lens is too close to the disc).
On the other hand, if a distance between the objective lens and a disc is longer than a predetermined focal distance, a reflected light having a vertically arranged elliptical shape will come to the light-receiving area, and thus a difference between the output signals of different sections of the light-receiving area may be calculated, thereby producing another focus error signal (indicating a so-called "rear pin state" in which the lens is too far away from the disc).
In this way, the level of each focus error signal is detected one by one, and the position of the objective lens is adjusted constantly so that a true circular pattern of a reflected light beam will constantly come to the central position of the light-receiving area, thereby maintaining a correct focusing operation.
On the other hand, a spot size method may be carried out in a manner shown in FIGS. 1A and 7B. As shown in the drawings, a reflected light beam obtained by reflecting a recording beam or a reading beam on a recording surface of a disc can be detected by photodetectors PD1 and PD2 each having a plurality of symmetrically arranged areas for receiving a light beam, so that it is allowed to calculate a difference between the levels of output signals fed from different detecting areas, thereby producing a focus error signal FE.
In detail, the photodetector PD1 shown in FIG. 7A has two coaxially arranged light-receiving areas A1 and A2. A difference between the levels of the signals fed from the light-receiving areas A1 and A2 is calculated by a subtracter C1, thereby producing a focus error signal FE.
In fact, the focus error signal FE will change in its level so as to indicate a focused state or a deviated state (the lens is too close to or too far way from the disc), corresponding to sizes of reflected light patterns PT1, PT2, PT3. For example, if a reflected light having a pattern PT1 is incident on both the area A1 and the area A2, it is determined that the objective lens is just at its correct focusing position (a desired focused state has been obtained). If a reflected light having a pattern PT2 is incident on an area smaller than PT1 it is determined that the lens is too close to the disc ("front pin state"). On the other hand, if a reflected light having a pattern PT3 is incident on an area larger than PT1, it is determined that the lens is too far from the disc ("rear pin state"). In this way, the position of the objective lens may be continuously adjusted in accordance with a level change of a focus error signal, thereby constantly maintaining the objective lens at its correct focusing position so as to ensure a desired focused state.
Further, the photodetector PD2 shown in FIG. 7B has three light-receiving areas A1, A2 and A3 which are symmetrically arranged with respect to the optical axis of a reflected light beam. The levels of signals fed from the light-receiving areas A2 and A3 are added together in an adder C2 so as to obtain an added level. Then, a subtracter C3 is used to calculate a difference between the added level and the level of a signal fed from the light-receiving area A1, so as to produce a focus error signal FE. In this way, similar to an arrangement shown in FIG. 7A, the level of a focus error signal FE will change corresponding to the size of a reflected light spot incident on one or more of the light-receiving areas A1, A2 and A3, thereby indicating whether or not the object lens is in its correct focusing position. Therefore, the position of the objective lens may be adjusted in accordance with the level change of the focus error signal FE, so that the objective lens is allowed to be maintained at its correct focusing position, thereby constantly maintaining a desired focused state.
Actually, in an optical pickup device utilizing the astigmatism method, not only a focus error signal is generated in accordance with the detected signals fed from a 4-divided light-receiving area, but also a tracking error signal is generated as well (using a push-pull method).
Here, when an optical pickup device (employing the above described astigmatism method) is used to record information on or to read information from a high density recording medium such as DVD-RW (Rewritable DVD), a recording beam or a reading beam is required to trace the information grooves physically formed on the recording surface of a DVD-RW so as to perform a predetermined scanning. At this time, a beam spot pattern of a light reflected from a groove and from an adjacent land is detected by the four light-receiving sections. Then, a difference between the levels of detected signals fir on different sections is calculated so as to produce a tracking error signal containing a land positional information.
However, in order to detect such a tracking error signal with a high sensitivity, i.e., in order to obtain a tracking error signal having a high level, it is required that a recording light beam or a reading light beam be set so that the wavelength .lambda. of a laser light beam can satisfy an equation .DELTA.d=.lambda./8, wherein .DELTA.d represents a phase difference between a groove surface and a land surface (in other words, a land depth with respect to an adjacent groove).
However, in using a conventional optical pickup device employing an astigmatism method, when a tracking error signal is obtained with a high sensitivity, the tracking error signal will partially leak into a focus error signal, hence undesirably decreasing a detection precision in a process of detecting a focus error signal.
On the other hand, when using a spot size method shown in FIG. 7A and FIG. 7B, there will occur a problem shown in FIG. 8A and 8B. Namely, as shown in the drawings, when an incident position of a reflected light is deviated from its optical axis, a detection precision in a process of detecting a focus error signal will be greatly decreased. Since the detection precision for detecting a focus error signal greatly depends upon the precision of an optical system itself, it is required to employ a high precision optical system which is usually very expensive. Further, if a recording medium is a DVD-RW capable of recording information with a high density, a focus servo operation is required to be conducted at a further high precision, this however has been proved difficult in industrial production when manufacturing an optical pickup.